Tunable magnetron



Nov. 10, 1953 F. H. CRAWFORD TUNABLE MAGNETRON Filed March 5, 1946 M N Il/l/l/l/l/l/A FIG.2

TUNING SETTING FINGER 3 4 POSITION 0F COUPLING LOOP INVENTOR.

FIG.3 FRANZO H.CRAWFORD A 7' TOR/VE Y Patented Nov. 10, 1953 UNITED STATES PATENT OFFICE.

TUNABLE MAGNETRON Franzo H. Crawford, Williamstown, Ma'ss., as-

signor to the United States of America as represented by the Secretary of .War

Application March 5, 1946, Serial No. 652,188

Claims. 1

This invention relates to electrical apparatus and more particularly to improvements in magnetrons.

One type of magnetron radio frequency oscillator is the so called squirrel cage magnetron. The squirrel cage magnetron is generally constructed as a substantially cylindrical cavity resonator within which is placed an axially aligned cathode structure. Radio frequency power is coupled out of the magnetron by an output coupling 100p disposed through the wall of the cavity resonator. A magnetic field is maintained parallel to the magnetron axis in the usual fashion.

Cylindrically arranged about the cathode and within the cavity resonator is a ring of interleaving anode fingers. The term squirrel cage is derived from the appearance of the cylindrical ring formed by the interleaving anode fingers. In the basic design, adjacent anode fingers are connected at one end thereof to one of two opposite annular anode halves which are connected in turn to the opposite end closures of the cavity resonator. In improved designs, the anode finger connections are modified slightly in order to improve the efficiency of the magnetron; this modification utilizes the so called phasereversing anode fingers which are placed at two substantially diametrically opposite points, 90 either side of the output coupling loop, in the ring of interleaving anode fingers. The hasereversing finger may have various forms; in one embodiment, the two adjacent anode fingers at each of the aforementioned diametrically opposite points are connected to the same anode half, the remainder of the fingers being rearranged to be alternately connected to opposite anode halves; in another embodiment, these two pairs of two adjacent fingers are each replaced by a single wide finger having a circumferential dimension substantially that of two ordinary anode fingers plus the spacing between fingers; in a third embodiment, the two pairs of fingers are omitted entirely. Further information relative to the phase-reversing anode finger may be had by reference to the patent application of Franzo I-I. Crawford, Milton D. Hare and Carl G. Hok for Magnetron, S. N. 652,186; filing date March 5, 1946.

Laboratory tests indicate that for the desired mode of oscillation, a standing Wave of radio fret quency potential is set up around the circu'n rference of the ring of interleaving anode fingers. The magnitude and relative phase of the radio frequency voltages in this standing wave may be represented by a single sine wave around the 360 of the circumference.

The present invention is designed to eliminate certain moding difficulties. Investigation shows that there are two modes of substantially the same frequency which tend to be excited for the same anode voltage and magnetic field. Each of these two modes has the aforementioned sine Wave distribution about the circumference of the ring of anode fingers, the two sine waves being out of phase. These two modes will be rereferred to as the -A and B modes. The A mode which is the desired mode has two Voltage nodes, one at each of the two phase-reversing anode fingers and two voltage loops, voltage maxima, 90 away. One of the two voltage loops for the A mode occurs at the output coupling loop. The B mode distribution is 90 out of phase with the A mode distributiona The B mode has a voltage loop at each of the two phase-reversing anode fingers and a voltage node, that is one of its two voltage nodes, at the output coupling loop. The existence of the B mode is undesirable because it represents a power loss. In addition the fact that the B mode is so lightly loaded by the output loop allows it to build up to considerable amplitude.

Accordingly, an object of the, present invention is to shift the frequency of the B mode so that it will not be excited by the same values of magnetic field and anode voltage as is the A mode.

Other objectsand advantages of the present invention will be apparent during the course of the following description.

In essence this invention utilizes a means for adding or substracting capacity at the voltage nodes of the A mode, that is at the phase-reversing fingers. This added or substracted capacity then appears at the voltage loops of the B mode, but at the voltage nodes of the A mode. Consequently, the frequency of the B mode will be shifted while that of the A mode will be relatively unchanged. In one form of adding this capacity, a capacitive finger is disposed adjacent to, but

separated from, each of the two wide phasereversing fingers and attached to the opposite anode half to which its corresponding phasereversing finger is attached.

In the accompanying drawing forming a part of this specification,

Fig. 1 is a symbolic representation of the anode finger and capacitive finger connections for one embodiment of the present invention,

Fig. 2 is a simplified cross section taken on the line 2-2 of Fig. 1,

Fig. 3 shows the distribution of radio frequency potential for the A and B modes around the circumference of the ring of interleaving anode fingers and Fig. 4 shows the tuning curves for the A and- B modes before and after the inclusion of the capacitive fingers.

Fig. 5 is a simplified isometric view, partly broken away, of a squirrel-cage magnetron of the type represented schematically in Figs. 1 and 2.

In Fig. l, numerals l-l8 designate individual anode fingers forming a squirrel cage ring of interleaving anode fingers. Numerals 3-4 and l2-l3 designate wide phase-reversing anode fingers. The anode fingers having the X mark indicate fingers connected to the lower anode half, while the blank anode fingers indicate fingers connected to upper anode half 20, see Fig. 2. Numeral 22 indicates the approximate position of the output coupling loop. Although the output coupling loop is shown adjacent to anode finger 8, it may be placed diametrically opposite, adjacent to finger I1.

Adjacent to but insulated from the wide fingers 3-4.and l2l3 are two capacitive fingers 23 and 24,. these capacitive fingers being connected to the opposite anode half to which their respective wide anode fingers 3-4 and l2 -i3 are connected.

Fig. 2 shows a simplified cross sectional View taken on the line 2-2 of Fig. 1 and more clearly illustrates this arrangement. Capacitive fingers 23 and 2-4 are spaced from upper anode half by means of spacers 25. spacers 25 may be a continuous ring if desired.

In Fig. 3, are shown the sine wave distributions of radio frequency potential for modes A and B. The wide fingers 34 and 12'l3 are located at the voltage nodes on sine wave A. The position of the coupling loop is located190 away at a voltage maximum in sine wave A. Sine wave B is 90 out of. phase with sine wave A. By capacitively loading the oscillations of modes A and B at the wide fingers 3-4 and I2l3, mode A is comparatively unafiected since the capacity is placed at a voltage node. Mode B however, is very much afiected since the capacity. is placed at its voltage loop.

In Fig. 5 numeral designates a cavity resonator, partly broken away to reveal the anode structure. Cavity resonator 30 may have any of the customary cavity resonator shapes, and in general will be a figure of revolution circularly symmetrical about an axis 3l--3i. The cavity resonator 33 has two opposing flexible diaphragm-like walls or end closures 32 and 33.

Axially aligned along axis 3l--3I' are a filament 34 and cathode 35. A- ring of interleaving anode posts or figures 40 is concentrically arranged about cathode 35 and within cavity resonator 30. Alternate anode fingers are generally connected to end closure 32 by being at-- tached at one end to annular anode'section 3B and end plate-38'fixedto-wall 32. Theother anode fingers are connected to the end closure 33 by being attached at one end to annular anode section 37 and end plate 39 fixed to wall 33. At diametrically opposite points of anode section 36 there are two phase-reversing anode fingers having a greater width than the other anode fingers, as is clearly shown in Fig. 1. A capacitive finger 23 is mounted on anode section 31, but spaced therefrom in the radial direction. A similar capacitive finger 24 is diametrically opposite finger 23. The magnetron is tuned by flexing walls 32 and 33, as indicated by the arrows, so as to move the anode sections 36 and 31 together or apart and thereby vary the capacity between the anode sections.

The radio frequency output of the magnetron may be taken off in the customary manner by any suitable coupling, not shown, to cavity resonator 30.

Fig. 4 shows the efiect on the tuning curve. Curve A represents the tuning curve of wavelength vs. tuning setting for mode A, while curve B represents the same for mode B without the use of the capacitive fingers. By using the capacitive fingers the tuning curve for mode B is shifted to curve B. Hence for a value of magnetic field and anode voltage necessary to excite mode A, mode B will not be excited. Fig. 4 clearly shows the efiicacy of the present invention in eliminating the B mode.

A relative reduction in capacity at the voltage nodes of the A mode may be afiected by the use of capacitive fingers adjacent to all the anode fingers except the two wide fingers. This shifts the B curve below the A curve of Fig. 4.

Many modifications of my invention as described above and illustrated in the accompanying drawing will be apparent to those skilled in the art, and, therefore, the specific embodiment of the invention disclosed in this application should only be considered illustrative of the principles of my invention.

What is claimed is:

l. A magnetron anode including two annular anode sections disposed coaxially and opposite one another, a ring of interleaving anode fingers mounted circumferentially with substantially uniform spacing about the axis of said annular anode sections, two substantially diametrically opposite fingers in said ring being wide anode fingers having a circumferential dimension substantially that of two ordinary anode fingers plus the spacing between fingers, adjacent fingers in said ring being connected at one end thereof to opposite annular anode sections, and means reactively coupling one of said wide anode fingers to the anode section opposite the anode section to which said wide finger is connected.

2. A magnetron anode as in claim 1 in which W said means includes acapacitive finger disposed adjacent to one of said wide fingers and connected to the anode section opposite the anode section to which said wide finger is connected.

3. A magnetron anode as defined in claim 2, including a capacitive finger disposed adjacent the other of said wide fingers and connected to the anode section opposite the anode section to which said other wide finger is connected.

4. A squirrel cage magnetron including two opposed surfaces, a ring of circumferentially disposed anode fingers, alternate fingers being connected at one end thereof to one of said surfaces, and the other'fingers being connected at one end thereof to the other of said" surfaces, aplurality of said alternatefingers having a. given arcuate extent and at least one of said alternate fingers being a phase reversing finger having an arcuate extent which is different from said given arcuate extent, all of said other fingers having said given arcuate extent, and means connected to said other surface and positioned adjacent said phase reversing finger for reactively coupling said phase reversing finger to said other surface.

5. A squirrel cage magnetron as set forth in claim 4, wherein said alternate fingers include two phase reversing fingers, said phase reversing fingers being diametrically opposite to each other.

FRANZO H. CRAWFORD.

References Cited in the file of this patent Number UNITED STATES PATENTS Name Date Dallenbach Aug. 30, 1938 Hollmann Jan. 17, 1939 Morton Oct. 15, 1946 Hansell July 29, 1947 Crawford et a1 Apr. 25, 1950 

